president obama is planning to visit boston on thursday for an interfaith service for those systems. sally kidd during this with the latest. officials are asking whether they're doing enough. experts say there's only so much that can be done. horrific explosions are prompting cities across the country to reevaluate security at public events. >> i did not think anyone expected bombs to go off at the end of the marathon. >> they're taking a second look at security. including two races that will go on this weekend. >> there are certain events that will be open to us by their very nature. marathon is 26 miles long. points of vulnerability. additional training and pattern recognition could help. of stopping ality terrorist using all those things i mentioned, the trailing, this surveillance is still probably 100 to%, maybe even more than that. officials are looking at boston and make adjustments where appropriate. police say they're counting on the public to be vigilant. if you see something suspicious, reported. international officials are taking precautions. the winter olympics' security